2021 in numbers: Rainfall

An HT analysis shows that 2021 has been unique in following only part of this long-term trend at the country-level. Here are three charts that show how

The long-term pattern of rainfall in India is that while the total amount of rainfall is declining, although there was a bump in the last decade. It is also becoming more intense and skewed. An HT analysis shows that 2021 has been unique in following only part of this long-term trend at the country-level, a pattern likely to hold when the year ends. Here are three charts that show how.

According to gridded data from the India Meteorological Department (IMD), India has received 1236.6 mm of rainfall up to 8:30 AM on December 21(L. Anantha Krishnan)

According to gridded data from the India Meteorological Department (IMD), India has received 1236.6 mm of rainfall up to 8:30 AM on December 21. Unlike the long-term trend of declining rainfall, this is the 23rd highest amount of rainfall since 1901, the earliest year for which this data is available. To be sure, in 2019 and 2020 total rainfall in this interval was about 50 mm more than this year. These years are ranked 9th and 8th respectively. The rainfall this year also falls just within the normal range. The IMD classifies all India level full year rainfall as normal if it has not deviated more than 10% from the Long Period Average (LPA) or the average of rainfall in the 1961 to 2010 period. This year’s rainfall so far is 9.1% above the LPA.

The headline number however hides the uneven distribution of rainfall through the year. Weekly or seasonal rainfall is classified as normal if it is within 19% deviation from the LPA. Only four months in the year – January, April, June and July – received normal rainfall.

While the pre-monsoon period of January-May received normal rainfall overall, the cumulative monsoon for the year was running deficient – more than 19% less than the LPA – up to April. This is because while February and March received much less rainfall than the LPA for those months, May received much higher rainfall than the LPA. The rainfall in February and March was the 6th and 20th lowest since 1901 while May rainfall was the highest ever.

The southwest monsoon period of June-September, when India receives most of its rainfall, similarly received normal rains (2.7% more than the LPA) overall, but was characterised by high rainfall in the first and last month and dry months in between. June and September rainfall was 27th and 8th highest since 1901 while July and August rainfall was 30th and 7th lowest.

The October-December north-east monsoon period on the other hand, when the peninsular region gets rainfall, has been of excess rainfall (mor than 19% deviation from LPA) throughout. The three months, with December shortened to 21 days for comparison, are ranked 21st, 7th and 31st respectively.

Was the rainfall this year one of the most intense? Not necessarily in its composition, although rainfall of all kinds of intensities are among the top 30 highest amounts since 1901, a reflection of the fact that total rainfall itself is ranked 23rd. Less than 7.5 mm rainfall in 24 hours in a grid – a box bound by two latitudes and longitudes 0.25 degrees apart – is classified as light rainfall. It is called moderate when between 7.5 mm and 35.5 mm, called heavy when between 35.5 mm and 244.5 mm, and called extreme when above 244.5 mm. Moderate rainfall generally has the largest share in total rainfall – close to half of all rainfall – while extreme rainfall has the smallest (less than 1%). The share of extreme rainfall this year so far is 0.768%, which is the 26th highest share since 1901. This suggests that this year’s rainfall did have one of the highest shares of disaster-potential rainfall. However, the share of heavy intensity rainfall (37.83%) was only the 68th highest since 1901, somewhat middle-ranked.